1. Field of the Invention
The present invention relates to an automatic transmission control system, and, more particularly, to a control system for an automatic transmission having a plurality of fiction coupling elements which are selectively coupled and uncoupled with hydraulic pressure controlled by means of duty solenoid valves to place the automatic transmission in desired gears.
2. Description of Related Art
Typically, an automatic transmission for an automobiles has a torque converter and a transmission gear mechanism driven by the torque converter. Each such a transmission gear mechanism includes a plurality of friction coupling elements, such as clutches and brakes, which are selectively coupled and uncoupled so as to place automatically the automatic transmission into desired gears according to driving conditions. Selectively coupling and uncoupling these friction coupling elements is performed by means of a hydraulic pressure control system including a regulator valve for regulating a pressure discharged from an oil pump to a predetermined level of line pressure, a manual shift valve for shifting the automatic transmission to a manually selected range, and various shift valves for selectively locking and unlocking the friction coupling elements so as to automatically shift the automatic transmission to gears according to driving conditions. In resent years, in order for such a hydraulic control system to control appropriately an operating pressure for locking or unlocking each friction coupling element, duty solenoid valves capable of being electrically controlled are used. One example of such a hydraulic control system is that described in Japanese Unexamined Patent Publication No. 1-299351. The technique makes utilization of three duty solenoid valves for controlling operating pressures for friction coupling elements, such as a C-1 clutch locked in first to third gears, a C-0 clutch locked in third and fourth gears, and a B-1 brake locked in second and fourth gears, respectively.
While the hydraulic control system has the advantage that supplying and discharging the operating pressure is timely and accurately controlled during, in particular, gear shifting without being accompanied by a significant shift shock, realizing an improved feeling of gear shift, nevertheless, there occurs a shift shock and a delay in response of engine braking in low-speed (S) range. This is because, in the prior art hydraulic control system, electric control is effective only to the duty solenoid valves which control operating pressures to selectively lock and unlock the three friction coupling elements in drive (D) range. For example, an operating pressure to the low-reverse brake (B-2 brake) which is locked for the first gear in the low speed (L) range where engine braking is needed is only supplied or not supplied through the manual shift valve. When making a shift to low-speed (L) range, where engine braking is needed, during running in second gear in drive (D) range, locking the low reverse brake does not always take place at a best timing, there still occurs a shift shock and a delay in response of engine braking in low-speed (S) range.